1. Field of the Invention
The present invention relates to an improvement in a mask defect inspection apparatus for inspecting a defect in a mask having a pattern (for example, break, thinning or attachment of foreign substance) formed on or in a sample of a photomask or a glass wafer used for fabrication of semiconductors.
2. Description of the Related Art
Heretofore, in a mask defect inspection apparatus for inspecting a defect in a pattern formed on a photomask used for fabrication of semiconductors, there has been known an apparatus in which a chip having a pattern drawn is observed using one detection optical system and an observed image is then compared with a standard image which corresponds to design data to inspect the defect in the mask.
In addition, as the conventional mask defect inspection apparatus of this kind, there has been also known an apparatus in which a reflection-illumination optical system and a transmission-illumination optical system are provided as a detection optical system so that an inspection of defect in the mask may be made with higher sensitivity (for reference, see JP-A 10-97053).
With a development in miniaturization of semiconductor devices, a wavelength of illumination light used for exposure and inspection is increasingly shortened, and deep-ultraviolet rays (DUV) are being used as the illumination light in the mask defect inspection apparatus in the recent years.
On the other hand, since the detection optical system used for the mask defect inspection apparatus has performance equivalent to that of a high-resolution microscope and uses an objective lens of high numerical aperture (high NA), such a detection optical system has high resolving power but shallow depth of focus. As a result, the detection optical system has properties that the depth of focus becomes shallower as the wavelength of illumination light becomes shorter.
Accordingly, the conventional apparatus of this type performs the inspection by finely tuning a focal plane of the detection optical system as well as by switching over an inspection carried out with transmission-illumination light and an inspection carried out with reflection-illumination light, so that it may be identified whether an detected defect is mainly due to the residue of the film-forming material or attachment of the foreign substance onto the mask.
However, while the conventional mask defect inspection apparatus of this type is on a trend that the shortening of wavelength is called for and the depth of focus becomes shallower, there is a tendency that a size in film thickness of the pattern of the mask as an object to be inspected is becoming thicker. Moreover, in addition to a demand for strict accuracy in finely tuning the focal plane for both the inspection using the transmission-illumination and the inspection using the reflection-illumination, there has also been a demand for enlarging a range of tuning of the focal plane. Therefore, the conventional mask inspection apparatus is posing troubles in terms of a prompt inspection and identification of the defect in the mask.